This invention relates to ultrasonic diagnostic systems which measure blood flow by the Doppler technique and, in particular, to such systems in which the error due to the angle between the blood flow direction and the Doppler beam is automatically corrected by a vector processing technique.
Doppler ultrasound has long been used to quantify and image blood flow in the body. However, such measurements are angle-sensitive because ultrasonic blood flow velocity estimation is limited to the velocity component along the ultrasound beam axis. Although conventional Doppler analysis has great clinical utility, logical applications for Doppler velocity measurements have proven unreliable when existing instruments are used in an attempt to quantify the velocity of blood flow through an artery or vein. The ultrasonic Doppler technique will accurately measure flow which is in line with (parallel to) the axis of the Doppler beam. However, it is usually not possible to orient the Doppler beam for this alignment, particularly for superficial vessels which are substantially parallel to the skinline. This gives rise to an error in the measured velocity which is a function of the angle between the Doppler beam and the direction of flow. The conventional way to account for this error is for the clinician to set a cursor in alignment with the axis of the vessel, then use the angle between this cursor and the Doppler beam to correct the measurement. The papers xe2x80x9cPhysiological Pulsatile Flow Experiments in a Model of the Human Aortic Archxe2x80x9d by T. L. Yearwood and K. B. Chandran, publ. in Journal of Biomechanics, vol. 15, No. 9, pp 683-704, (1984) and xe2x80x9cHemodynamics of the Normal Human Carotid Bifurcation: In Vitro and In Vivo Studiesxe2x80x9d by D. N. Ku, D. J. Phillips, D. P. Giddens, and D. E. Strandness, publ. in Ultrasound in Medicine and Biology, vol. 11, pp 13-26, (1985), as well as the more recent paper xe2x80x9cSpiral laminar flow in Arteriesxe2x80x9d by P. A. Stonebridge and C. M. Brophy, publ. in The Lancet, vol. 338, Nov. 30, 1991 at pp 1360-1361, have shown that the normal flow in most arteries is helical, not parallel to the vessel axis, due to the effect of the bends and bifurcations in the artery. Although blood does flow parallel to the walls of an artery in the regions near it walls, this does not mean that velocities are parallel to the axis of the artery throughout the vessel. Thus, it is not possible, using the cosine of the angle between the Doppler ultrasound beam and the vessel axis, to determine correctly the magnitude of the velocity from the vector component projected onto the ultrasound beam. Moreover the hemodynamics of the flow get even more complex in pathological arteries, due to atherosclerotic obstructive disease. Other diseases such as deep venous thrombosis of the leg (abnormal proximal venous obstruction) or venous valvular incompetence of the leg (abnormal venous reflux) or aortic valve stenosis, or cardiac valve regurgitation can generate complex flow patterns as well, for which the vessel-aligned cursor does not yield an accurate velocity measurement.
In accordance with the principles of the present invention, a technique is presented by which the angle of the blood flow or tissue motion is automatically calculated and displayed in real time to the user in pulsed wave Doppler. It is thereby possible, using the cosine of this calculated angle to determine correctly the magnitude of the velocity of the blood flow or tissue motion. The inventive technique utilizes an ultrasonic transducer array in a crossbeam configuration capable of resolving two orthogonal components of the velocity vector. Velocity vectors determined by the two beams are used to resolve a true velocity vector, and this vector is used to automatically set the orientation of the motion cursor on the ultrasonic image of the vessel. The clinician may accept the automatic cursor placement as the direction of flow or motion, may manually alter its orientation, or may compare velocities calculated from the automatically placed motion cursor with those calculated from a cursor manually oriented by the clinician. Other techniques for determining the direction of fluid or tissue motion are also described.